This invention relates to a method of controlling fuel supply for an internal combustion engine at idle, and more particularly to a method of this kind which is intended to stabilize engine rotational speed when the engine is operating in an idling condition.
It is desirable that an internal combustion engine rotates stably at a desired idling speed in accordance with a condition in which the engine is operating in an idling region. However, change in engine load can actually cause it difficult to keep the engine rotational speed at the desired idling speed.
It is conventionally known e.g. from Japanese Provisional Patent Publication (Kokai) No. 58-176424 to increase or decrease the quantity of fuel supplied to the engine during idling, in a manner responsive to the difference between the actual engine rotational speed and a desired idling speed.
According to the above known method, the actual engine rotational speed is detected and compared with the desired idling speed. If the actual engine rotational speed is below the desired idling speed, the fuel quantity is increased by a correction value corresponding to the difference. On the other hand, if the actual engine rotational speed is above the desired idling speed, the fuel quantity is decreased by a correction value corresponding to the difference.
However, according to the known method, if the load on the engine suddenly changes due to an external disturbance e.g. increased or decreased electrical load on the engine, etc., the engine rotational speed can suddenly drop or rise across the desired idling speed (overshooting), often resulting in hunting of the engine rotation. To be specific, when the engine rotational speed starts suddenly increasing toward the desired idling speed due to a sudden decrease in the engine load, the fuel supply quantity is corrected to an increased value while the actual engine rotational speed is below the desired idling speed. As a result, the engine rotational speed can largely surpass the desired idling speed. On the contrary, when the engine rotational speed suddenly decreases toward the desired idling speed due to a sudden increase in the engine load, the fuel supply quantity is corrected to a decreased value while the actual engine rotational speed is above the desired idling speed, so that the engine rotational speed can largely drop below the desired idling speed.
Thus, the known method is involved in a problem of hunting of the engine rotation due to a sudden change in the engine rotational speed.